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hi all,
I have a piece of foam that I like the shape of but I would like for it to be harder. I'd like to sue it as a building and don't want chunks breaking off.
is there a trick to do this rather then the only thing that I can think of, covering it in styrene?

Various foam coats are the easiest way to harden it. Some are a 2 part urethane like resin, others are a thick latex type paint. Quite often I use foundation paint for covering foam, cheap and readily available.

If you don't mind the texture, paper mache can work. Pretend you're in kindergarten and slop on some tissue/toilet paper/paper towels wetted with PVA glue.

If you want a smooth finish you might be better served working with either foam core or plasticard sheet to begin with.

Plaster impregnated bandage works.

Once the initial application sets, you rub it down with a wet hand to re-wet the plaster slurry and cover the fibres.

Or fibreglass (both the glass-fibre cloth AND the resin used to actually give it shape). This does require more in the way of safety gear, though - as it is can be a hazard to your health if you do it wrong or badly.

show it some dirty foam pics? http://static.panoramio.com/photos/large/46662526.jpg

I would try a really liquid super glue worked for me "small scale"

OgreChubbs wrote:
show it some dirty foam pics? http://static.panoramio.com/photos/large/46662526.jpg

I would try a really liquid super glue worked for me "small scale"

DO NOT TRY THIS! Super glue melts foam. While I don't doubt that the poster is telling the truth about whatever they used it on, it strongly depends on the type of foam, as most will melt when touched with cyanoacrylate.

The options above are much, much safer.

I've used the blue shop towels from an Auto store with diluted Plaster of Paris. Apply just like Paper Mache and it works well.

ty all very much for the ideas.
searching around a bit it seems like slightly watered down wood glue is a popular idea.
has anyone had any luck trying that?

PVA will help seal the foam against solvents (like those in aerosols, letting you spray paint it), but it doesn't make it all that much harder. For that, you'd need something... well, hard. Terranscapes reviews a number of foam coats (both plastic- and plaster-based) on his Youtube channel, if you're interested.

Personally, I tend to use what's handy, so I've gone with spackle, primarily. Mixing or coating it with PVA results in a somewhat more plastic surface, which helps reduce chips and cracks.

 usernamesareannoying wrote:
ty all very much for the ideas.
searching around a bit it seems like slightly watered down wood glue is a popular idea.
has anyone had any luck trying that?

I've never tried wood glue (PVA) by itself, but I have done paper mache with watered down wood glue. It works fine, I have stuff from 10 years ago that is still holding up. I can't remember exactly what I used, I think tissues and watered down PVA, it was definitely PVA because it's what I had on hand. Plaster works fine too, I'd just use whatever you think will be easier and/or you already have on hand and/or will give you the texture you want.

What kind of foam is it? The yellow and blue foam tend to be pretty hard. Pink foam I think is a bit softer but still sturdy. And white packing foam is soft and very weak. I'm guessing your talking about white packing foam, though I could be wrong. But I mixed up wall putty ( plaster ) with pva glue aswell as the water. This helped to harden the foam but stopped the dried plaster mix cracking and chipping if your pressed on it. Another idea I just thought of is tile glue. That stuff is tough when dry. Maybe try some of that if you have it laying around. It's also gritty so great texture.

its not that terrible foam that breaks apart and then flakes all over the place. its actually pretty dense.
I'd say its probably as hard as or at least close to pink foam although it seems like it might be a bit more porous.